Organic light-emitting diodes (OLED) generate light by re-combination of electrons and holes, and emit light when a bias is applied between the anode and cathode such that an electrical current passes between them. The brightness of the light is related to the amount of the current. If there is no current, there will be no light emission, so OLED technology is a type of technology capable of absolute blacks and achieving almost “infinite” contrast ratio between pixels when used in display applications.
In many conventional configurations, the OLED in the sub-pixel of a display is driven by an analogue drive transistor (drive TFT), which is in series with the OLED. The amount of current supplied to the OLED is related to the voltage on the gate of the drive TFT. The gate voltage is normally stored on a capacitor. The drive TFT device characteristics may vary due to manufacture processes or stress and aging of the drive TFT during the device operation. Accordingly, even if the gate voltage is the same between two different drive TFTs, the amount of current delivered by the drive TFT to the OLED may vary by a large amount, causing an unwanted variation in the brightness of the OLED sub-pixel. In addition, OLED device characteristics may vary due to manufacture processes, stress and aging during the operation of the OLED. For example, the threshold voltage of the OLED for light emission may change. Conventional circuit configurations, therefore, often include elements that operate to compensate for at least some of these component variations to achieve an OLED display with more uniform brightness between sub-pixels.
Accordingly, there are various methods proposed to compensate the drive TFT and OLED variations. Normally, such methods use a circuit configuration having several transistors. The size required by many of these circuit configurations may not be suitable for high resolution (e.g. high pixels per inch or ppi) display applications, in which each subpixel must occupy only a small area.
Conventionally, an OLED is programmed either by current programming or voltage programming. An example of OLED programming is a charge-based programming method, such as disclosed for example in U.S. Pat. No. 5,714,968 (Ikeda, issued Feb. 3, 1998), which uses one digital switch transistor and one storage capacitor. In such configuration and method, the transistor is connected to a data voltage line. When a control signal is applied to the gate of the transistor corresponding to the on state, the data voltage is applied to an OLED device through the transistor, and also to the storage capacitor connected in parallel with the OLED device. With application of the data voltage, the OLED begins to emit light while the capacitor is charged. When the gate voltage is switched to place the transistor in the off state, the data voltage is disconnected, but the capacitor continues discharging through OLED. The OLED, therefore, continues emitting light until the voltage from the capacitor charge is below the threshold voltage of the OLED.
Such a configuration that operates by charge-based programming lacks the analogue drive transistor of other conventional configurations, and thus variations of drive transistor properties will not be applicable to performance. Conventional charge-based programming configurations such as described above, however, do not compensate the OLED variations. In addition, conventional charge-based programming configurations employ a constant or bias current source applied to the OLED. This type of current source may be difficult to realize in practice when the current source must supply the current to a large number of sub-pixels in a column, such as in a display device. In particular for low current circumstances or applications, the drive speed could be detrimentally slow.